1. Field of the Invention
This invention relates to a method for manufacturing a rubber sheet having electrically insulating properties and thermally radiating properties.
2. Description of the Prior Art
Heat-producing electronic components such as power transistors or thyristors usually generate heat in their operation. As a result, when these electronic components are used for a long period of time, their properties deteriorate due to the heat and they occasionally fail. For this reason, an electric-insulting and radiating sheet which exhibits both electric insulation properties and thermal conductivity is placed between the above-mentioned heat-producing electronic parts and constructed with radiator fins (or metal radiator plates) in order to provide the heat-producing electronic part in such a way that the heat generated will be radiated.
Conventional radiating sheets are usually mica or a polyester film coated with grease. However, the grease can evaporate and is degraded in long-term use with the result that its radiating characteristics tend to degrade and it causes complications in operation. In addition, mica and polyimide films are hard and their surfaces are not adhesive. These are the drawbacks of conventional radiating sheets.
It is known that a silicone rubber radiating sheet alone has recently been used without the use of grease. However, this thin silicone rubber radiating sheet exhibited the drawback of low mechanical strength with the result that it presented problems such as tearing during installation. For this reason, the use of glass cloth has been proposed to reinforce the above-mentioned sheet to give high reliability. A conventional method for the production of an insulating and radiating rubber sheet reinforced with such a network insulating material (glass cloth) is to dip the network material into a nonfluid (solid) peroxide-vulcanized silicone rubber compound dissolved in a large amount of solvent and the compound cured to a radiating rubber. However, the nonfluid rubber compound is not very soluble and a long time is required for dissolution and the solid component concentration in the resulting solution is low. Due to this, the number of immersions must be increased and the process becomes very complicated. Furthermore, this process poses safety and hygiene problems due to the use of a large amount of an organic solvent. Also, the above-mentioned insulating and radiating rubber sheet exhibits the drawback of ready peeling of the silicone rubber from the network insulating material under repeated flexural loading.